Kyle King made $8 an hour when he started working as a cashier at the Burger King across from Boston Common. Nine years later, he is up to $8.15, logging fewer hours and living with his brother in Roxbury because he can’t afford a place of his own.

King’s worsening economic condition has prompted him to make a bold decision that could cost him even more money: He plans to skip his scheduled Thursday afternoon shift at the chain’s Tremont Street restaurant.

But King isn’t quitting. Instead, he is taking part in a nationwide demonstration of fast-food employees demanding $15-an-hour wages and the right to unionize. Thousands of workers in 50 cities are expected to take part in the one-day strike. In Boston, as many as 200 fast-food employees are expected to form rolling picket lines outside nine chain restaurants — including Burger King, McDonald’s, and Dunkin’ Donuts — culminating in a rally on Boston Common.

King, 45, said he realizes low-skilled workers like him are easily replaceable and that he could be fired over Thursday’s act of defiance. But he is more worried that nothing will change for those who work behind fast-food counters unless more attention is called to their cause.

“I don’t know what I’ll lose first, lose my job or lose my sanity,” said King, who took computer maintenance classes but hasn’t been able to find a job in the field. “I feel just worthless.”

The fast-food employees’ uprising has been gaining momentum since a few hundred workers staged a one-day strike in New York in November, followed by demonstrations in Washington, D.C., Chicago, and several major Midwestern cities this summer.

Their movement is part of a mounting wave of actions by low-wage-workers, including picketing at more than 1,000 Walmart stores on Black Friday last November and an appeal by airport contract workers for better working conditions.

Pat Greenhouse/Globe Staff

Kyle King, who lives with his brother in Roxbury, says he will risk his job at Burger King to attend Thursday’s strike.

In Harvard Square last week, a four-person crew at the late-night delivery chain Insomnia Cookies walked off the job, calling for higher wages, health care, and the right to unionize. They were all fired, according to the Boston branch of the Industrial Workers of the World labor union.

Insomnia Cookies did not respond to a request for comment.

Fast-food industry representatives say large numbers of the demonstrators around the country have actually been union organizers, not restaurant employees.

Angelo Amador, vice president of labor and workforce for the National Restaurant Association, said many restaurants being targeted are run by franchisees who can’t afford to pay more money and remain profitable.

“We would imagine that a lot of the people would rather have a job than have these businesses close down,” Amador said.

Two years ago, New York’s Occupy Wall Street movement spread to other cities — including Boston — and reignited awareness of inequality in the nation’s economic structure, but didn’t have an apparent impact on workers’ paychecks. About a quarter of all US employees earn less than $11 an hour, and the bulk of new jobs added during the recovery from the recession have not been high-paying positions. Meanwhile, many companies are reporting improving profits and soaring productivity.

But salaries haven’t budged, according to the Center for Labor Market Studies at Northeastern University, and pay for low-wage earners has actually gone down.

The Obama administration is pushing to raise the federal minimum wage from $7.25 to $9 an hour, and in Massachusetts, legislators are considering a bill to raise the hourly minimum wage from $8 to $11 by 2015. Still, with the unemployment rate remaining stubbornly high and college graduates snapping up many entry-level jobs, employers have the upper hand, as the labor-force supply outstrips demand.

“We are going up against a $200 billion annual industry,” said Darrin Howell, deputy director of the local labor coalition MassUniting, which is organizing Thursday’s demonstrations in Boston. “The reason they’re able to have profits that high and make the money that they do is the fact that they have a labor force. And that labor force’s hourly wages are less than some of the ‘value meals’ associated with these companies.”

The Service Employees International Union has also been providing logistical and financial support for the protesting fast-food workers, in coordination with local labor groups.

Several fast-food companies and restaurant owners did not respond to requests for comment. Michelle King, a spokeswoman for Canton-based Dunkin’ Donuts, said in a statement that the chain’s restaurants are “owned and operated by individual franchisees who are responsible for making their own business decisions, such as hours of operation, employee wages, and the benefits they offer their employees.”

Across the United States, 3.6 million people work at more than 200,000 fast-food restaurants. They are paid about $9 an hour, on average — often with no benefits and irregular schedules. But the perception that they are mostly teenagers cooking burgers and fries is no longer accurate, said James Green, a labor historian at the University of Massachusetts Boston.

Parents and retirees who cannot find employment elsewhere now sign up for such positions, including Maria de La Cruz, a 33-year-old Burger King cook in Dorchester. The single mother said she brings home $200 a week, and gets by with the help of food stamps, free health care from the state, and a subsidized apartment. De La Cruz, who is originally from the Dominican Republic, said her workload has doubled since the restaurant recently started making deliveries. More than ever, Cruz said, she and her colleagues deserve pay raises.

The restaurant franchise’s owner could not be reached for comment.

Green said the one-day work stoppage could cause enough chaos to affect customers, which would get the industry’s attention.

“If there was some consumer support for these protests, it might have an impact because these are industries that are very, very competitive and rely a great deal on consumer habits,” he said. “If these are disrupted, it could cause some changes in how people are paid or how they are treated.”

But Andrew Sum, director of the Center for Labor Market Studies at Northeastern University, said it is hard for low-wage workers to win significant concessions without a collective voice. And in an industry with high turnover and a mostly part-time workforce, he said, efforts to organize employees prove daunting.

“A lot of people are there because they can’t find a regular job,” Sum said. “It makes it difficult for people to want to invest their time and money to make their case if they don’t see themselves as being there for that long.”

Still, labor groups say they are heartened by the number of fast-food workers willing to speak out.

“The mark of a real movement is when workers begin to adopt a tactic on their own,” said Russ Davis, executive director of Massachusetts Jobs With Justice, a nonprofit advocacy group.

In an effort to make sure participants in Thursday’s walkout are not disciplined by their employers, MassUniting has called on community leaders, Boston City Council members, and mayoral candidates to walk protesters back into their restaurants Friday.

Jussara Dossantos, 19, who has worked for four years at a KFC in Roslindale, said she is willing to risk her job security to make a point.

“I think that $15 an hour would make sure that everybody has a better life,” said Dossantos, who helped MassUniting organize people for the demonstration. “It’s not going to happen tomorrow, but we’re going to make sure it happens somehow. They need to hear our voice. They need to hear about our struggles.”

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